Ball end mills have conventionally been used for the cutting of dies, etc., and a recent trend of making dies larger demands ball end mills to have cutting capability with higher speed, efficiency and precision as well as longer life. Further demand is to make ball end mills indexable for cutting cost reduction. To meet such demands, various indexable tools have been proposed.
JP 60-85815 A discloses, as shown in FIG. 20, a rotary cutting tool comprising (a) a holder 103 having four-direction perpendicular slits 101 in a tip end portion, and a sphere 102 bonded to a surface on the bottom side of the slits 101, (b) four fan-shaped cutting chips 104 detachably attached to the slits 101 of the holder 103, and (c) pressing plates 105 extending in an axial direction of the holder 103 and threadably engaging the holder 103 for fixing the cutting chips 104, each cutting chip 104 having a circular notch in a fan-shaped center portion, and the positioning of each cutting chip 104 being achieved by the circular notch attached to the sphere 102 in the holder 103. Though this rotary cutting tool is a four-flute ball end mill, the cutting chips 104 may be detached in high-speed cutting, because each cutting chip 104 is fixed by the screw 106 and the pressing plate 105. Also, because the positioning of each cutting chip 104 is achieved by contact between the circular notch of each cutting chip 104 and the sphere 102, it is difficult to fix the cutting chips 104 with high precision whenever exchanged, in view of the dimension precision of the circular notch and the sphere 102, the positioning precision and durability of the sphere 102, etc.
JU 54-29490 A discloses, as shown in FIG. 21, a boring tool comprising (a) a holder 204 comprising a pair of projections 202 sandwiching an inclined slit 201, and a threaded hole 203 penetrating both projections 202 through the slit 201, (b) a cutting edge chip 206 having a center through-hole 205, and (c) a fastening bolt 207 threadably engaging a threaded hole 203 of both projections 202 through the through-hole 205 of the cutting edge chip 206 for fixing the cutting edge chip 206, the cutting edges of the cutting edge chip 206 and a side surface of each projection 202 constituting a chip-evacuating groove 208 having an obtuse angle. Though the cutting edge chip 206 is inclined from the projections 202 in this boring tool, the through-hole 205 of the cutting edge chip 206 is not in contact with the threaded portion of the fastening bolt 207. The cutting edge chip 206 is fixed by decreasing the width of the slit 201 by fastening the bolt 207. However, this boring tool is so-called 2-flute, having insufficient cutting efficiency. In addition, such fixing method of the cutting edge chip 206 has such low positioning precision that it is not suitable as a finishing tool for conducting the three-dimensional cutting of die cavities, etc. at high speed and high precision.
JP 2001-121339 A discloses, as shown in FIG. 22, a throwaway end mill comprising a pair of projections 302 having an inclined slit 301 in a tip end portion, a cutting chip 303 attached to the slit 301, and a clamp bolt 304 penetrating the cutting chip 303 and threadably engaging a pair of projections 302, thereby having large chip pockets 305. However, because this throwaway end mill is so-called 2-flute, it does not have sufficient cutting efficiency. In addition, because the cutting chip 303 is not fixed by the clamp bolt 304, the positioning precision of the cutting edge chip 303 is low. Accordingly, this throwaway end mill is not suitable as a finishing tool for conducting the three-dimensional cutting of die cavities, etc. at high speed and high precision.
JP 11-239911 A discloses, as shown in FIG. 23, a throwaway-type cutting tool comprising a tool body 402 having a slit-shaped chip seat 401 extending in a diametrical direction, a throwaway chip 403 attached to the slit-shaped chip seat 401, and a screw 405 threadably engaging the tip end portion through a through-hole 404 of the throwaway chip 403. The chip seat 401 comprises a conical seat surface 406 slightly biased rearward in a portion corresponding to a conical head 405a of the screw 405 on one side surface, and a hole receiving a cylindrical portion 407 of the screw 405 on the other side surface. A side wall 408 of the hole is a reference surface for positioning the throwaway chip 403. When the screw 405 is fastened with the throwaway chip 403 attached to the slit-shaped chip seat 401 of the tool body 402, the conical head 405a of the screw 405 is pressed onto the conical seat surface 406, because the conical seat surface 406 is biased toward a bottom surface of the chip seat 401 by Δt. However, because this throwaway-type cutting tool is so-called 2-flute, it has insufficient cutting efficiency, not suitable for high speed cutting of die cavities, etc.
Japanese Patent 4531981 discloses, as shown in FIG. 24, a boring tool comprising a holder 502 having a slit 501 in a tip end portion; an insert 504 having an inclined tapered hole 503 and attached to the slit 501; and a clamp screw 505 having a tapered portion 505c between a head 505a and a threaded portion 505b, the tapered portion 505c penetrating the inclined tapered hole 503 of the insert 504, and the threaded portion 505b threadably engaging a threaded hole in the tip end portion. When the tapered portion 505c of the clamp screw 505 engages the inclined tapered hole 503 of the insert 504, the insert 504 is pressed onto a stopper surface of the tip end portion by a wedge function, thereby preventing the vibration of the insert 504 during cutting. However, because this boring tool is so-called 2-flute, it has insufficient cutting efficiency, failing to be used as a ball end mill for cutting die cavities, etc. at high speed.